Hide pipeline retry button for Merge Train/Pipelines

What does this MR do?

Merge Train pipelines cannot be retried. This MR is needed as the retry button appears against Merge Train pipelines, and if pressed it will cause an error.

This MR solves this issue my always returning false to the retryable method on both Ci::Pipeline and Build objects. In this case the UI will not display the retry button.

Both model and feature tests have been updated to verify these conditions.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #14853 (closed)